Lithographic printing is a printing process wherein a printing plate of which a portion of receiving ink, i.e. imaging portion, is composed of a hydrophobic resin composition layer having several micron thickness and a portion of receiving water, i.e. non-imaging portion, is composed of an exposed aluminum support of which surface is made hydrophilic by hydrophilic treatment, is employed and transferred onto a subjection to be printed. The photosensitive resin layer which constitutes the imaging portion is required to have lipophilic nature, print durability, chemical resistance, adhesion to a support, developability, high resolusion, coatability, storage stability and the like. Recently, in view of environmental problem or safety, it also has been desired to use an aqueous alkali developer in the step of forming printing plates, instead of organic solvent based developer.
Oil soluble diazo resin having diazonium salt has been used for the photosensitive resin layer of the lithographic printing, because it has good photosensitivity, lipophilic nature and storage stability. The diazo resin, however, has poor water solubility. In order to use an alkali developer, it is proposed to mix with an alkali-soluble binder resin (for example, Japanese Kokoku Publication 34929/1980 and 43890/1982). The alkali-soluble binder resin has high hydrophilic nature by increasing acid value, but this adversely reduces lipophilic nature after developing, resulting in poor resolusion.
Japanese Kokai Publication 32255/1989 proposes a process for reducing hydrophilic nature of the exposed portion by using a photosensitive resin having a carboxyl group which is decarboxylated upon exposure to light. The photosensitive resin, however, has poor photosensitivity and poor crosslinkability. Accordingly, the resulting lithographic printing plate has poor resolusion and poor print durability.